A well known optical design for a collector for X-ray applications is the type I Wolter telescope. The optical configuration of type I Wolter telescopes consists of nested double-reflection mirrors operating at grazing incidence angles.
More recently, a variation of the type I Wolter design already proposed for other applications, in which the parabolic surface is replaced by an ellipsoid, has found application for collecting the radiation at 13.5 nm emitted from a small hot plasma used as a source in Extreme Ultra-Violet (EUV) microlithography, currently considered a promising technology in the semiconductor industry for the next generation lithographic tools.
A simplified block diagram of an EUV lithography system is shown in FIG. 1 (PRIOR ART). The ultra-violet source 102 is normally a hot plasma the emission of which is collected by the collector 104 and delivered to an illuminator 106. The latter illuminates a mask or reticle 108 with the pattern to be transferred to the wafer 110. The image of the mask or reticle is projected onto the wafer 110 by the projection optics box 112.
FIG. 2 (PRIOR ART) depicts the conceptual optical layout of a known type I Wolter collector 104 for EUV plasma sources. In the nested Wolter I configuration, each mirror is a thin shell consisting of two sections (surfaces). Although many more nested mirrors in the collector optical system 104 may be illustrated, only two (202, 204) are shown. The radiation from the source 206 is first reflected by the hyperbolic surfaces 208, 210, then reflected by the elliptical surfaces 212, 214, and finally focused to an image or intermediate focus 216 on the optical axis 220. As in the type I Wolter telescope mentioned above, the elliptical (212, 214) and the hyperbolic (208, 210) surfaces share a common focus 218. For each of the mirrors 202, 204, etc. the different sections on which the surfaces 208, 212 are disposed may be integral, or may be fixed or mounted together.
In the aforementioned optical systems (for EUV and X-ray applications, mainly in the medical, astronomical and lithographical fields), a series of nested grazing incidence mirrors (mainly elliptical and Wolter I) are co-aligned, one with respect to the other, and all with respect to their mechanical support.
The alignment respect the mechanical support is very important because, when met, it assures that the entire optical system, when positioned in the machine (e.g. lithography system) for which it is designed, is automatically aligned, and no complex additional alignment systems and no additional alignment processes are required.
This plug-in capability is particularly useful when lithographic applications are concerned because the optical system must be replaced at frequent intervals, and because the machine downtime must be minimized during 7 h/day, 7 days/week mass production cycles.
A problem with known systems is how to provide mounting of the mirrors of the optical system with respect to each other and to the mechanical support, so that the mirrors are fixed to the mechanical support in aligned configuration.
A further problem with existing systems is that once the mechanical support is mounted in the machine, further, post-mounting, alignment of the optical system is usually required.